USG FLEX 500H stuck in boot loop

ABIDAR

Hi,

My Zyxel USG FLEX 500H is stuck in a boot loop and I can't get it to boot normally.

So far I have tried:

• Performing a factory reset.
• Connecting through the serial console.
• Switching to the other boot partition using:
  
• atcd 2
  atgo

(I also tried atcd 1 with the same result.)

Unfortunately, none of these methods solved the problem.

I have attached the complete boot log below.

Any help would be greatly appreciated.

Thank you.

boot log :

RAM test ........ done !

Loading kernel from FIT Image at 20000000 ...

Using 'conf@1' configuration
Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
Trying 'kernel@0' kernel subimage
Description: Linux Kernel
Created: 2026-01-29 4:07:04 UTC
Type: Kernel Image
Compression: uncompressed
Data Start: 0x200000d8
Data Size: 24254976 Bytes = 23.1 MiB
Architecture: AArch64
OS: Linux
Load Address: 0x08080000
Entry Point: 0x08080000
Hash algo: sha1
Hash value: aebe71fb5b6d2df9cd6a779cf8bb2173e69ad2ed
Verifying Hash Integrity ... sha1+ OK

Loading fdt from FIT Image at 20000000 ...

Using 'conf@1' configuration
Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
Trying 'fdt@0' fdt subimage
Description: Flattened Device Tree blob
Created: 2026-01-29 4:07:04 UTC
Type: Flat Device Tree
Compression: uncompressed
Data Start: 0x21721bd0
Data Size: 31395 Bytes = 30.7 KiB
Architecture: AArch64
Hash algo: sha1
Hash value: bdbc7c9667df66140a8ca6f84fae95a9b4de3472
Verifying Hash Integrity ... sha1+ OK
Loading Kernel Image

Starting kernel ...

Starting binary extension
mv_ddr: Marvell-devel (release) (Oct 11 2024 - 02:53:13)
SSCG_EN
dma memcmp pass
mv_ddr: completed successfully
Booting Trusted Firmware
BL1: Booting BL2
BL1: Booting BL31
▒

U-Boot 2019.10-10.3.4.0-4 (Oct 11 2024 - 02:52:20 +0000)

BootModule Version: V1.1.5 Oct 11 2024 02:52:20
DRAM: Size = 8192 Mbytes

Press any key to enter debug mode within 3 seconds.
................

Zyxel_Judy

Hi @ABIDAR ,

Thank you for the information you shared via private message.

As you mentioned, the firewall suddenly became stuck in a boot loop, and you're not sure whether this may have been caused by a power outage. We recommend connecting the firewall to a stable power source and attempting the firmware recovery again.

If the firewall remains stuck in the boot loop, please contact your reseller/ISP to arrange an RMA.

Zyxel_Judy

Hi @ABIDAR ,

Please go to MyZyxel to download the latest official firmware, then extract the file. Use the resulting .bin file for the USG FLEX 500H to perform a firmware recovery. You can refer to the firmware recovery steps here:

https://community.zyxel.com/en/discussion/28195/how-do-i-perform-firmware-recovery-on-usg-flex-h-series-models

ABIDAR

hi @Zyxel_Judy

Thank you for your suggestion.

I tested the recovery procedure as follows:

atkz -f -l 192.168.1.1

The command returns:

OK

Then I executed:

atgof

However, the firewall does not enter FTP recovery mode. Instead, it immediately starts booting again and falls back into the same boot loop.

ABIDAR

hi @Zyxel_Judy

I would also like to add that after running:

atkz -f -l 192.168.1.1
atgof

the message:

Connect a computer to port 1 and FTP to 192.168.1.1 to upload the new file.

never appears. The firewall immediately continues the normal boot sequence and enters the same boot loop.

As a result, I am unable to access the firewall through port 1 (or any Ethernet port) to perform the FTP recovery procedure.

Zyxel_Melen

Hi @ABIDAR

Please try this adjusted method:

STEP1.
Boot up the device and wait for the console message: 'Press any key to enter debug mode within 3 seconds. ..................' then enter the debug mode.

STEP2.
Enter atkz -f -l 192.168.1.1 This sets the FTP server IP address on the USG Flex 500H.

STEP3.
Set a static IP address on the PC to 192.168.1.XXX (e.g., 192.168.1.2).

STEP4.

Connect the Ethernet cable from the PC to port 1 of the USG Flex H model. Open the Command Prompt on the PC and enter ftp 192.168.1.1 to connect the USG Flex 500H and login to the device using the account admin.

STEP5.

On the PC, enter: bin

STEP6.

On the PC, enter: put C:\131ABZHC0.bin This uploads the .bin file to the USG Flex 500H.

C:\Users\employee>ftp 192.168.1.1
Connected to 192.168.1.1.
220-=(<>)=-.:. (( Welcome to PureFTPd 1.0.11 )) .:.-=(<>)=-
220-You are user number 1 of 50 allowed
220-Local time is now 09:49 and the load is 0.04. Server port: 21.
220-Only anonymous FTP is allowed here
220 You will be disconnected after 15 minutes of inactivity.
530 You aren't logged in
User (192.168.1.1:(none)): admin
230 Anonymous user logged in
ftp> bin
200 TYPE is now 8-bit binary
ftp> put C:\131ABZHC0.bin
200 PORT command successful
150 Connecting to port 1044
226-1786.8 Mbytes free disk space
226-File successfully transferred
226 1.605 seconds (measured here), 110.89 Mbytes per second
ftp: 186657587 bytes sent in 1.60Seconds 116807.00Kbytes/sec.
ftp>

STEP7.

Once the firmware update is successfully applied to the USG Flex 500H, the device will reboot automatically.

ABIDAR

Hi @Zyxel_Melen ,

I followed the procedure exactly as described:

1. Entered Debug Mode.
2. atkz -f -l 192.168.1.1The command returned OK.
3. Configured my PC with the static IP address 192.168.1.2/24.
4. Connected the PC directly to port 1 of the USG FLEX 500H.

However, I am unable to access the firewall via FTP. The message:

Connect a computer to port 1 and FTP to 192.168.1.1 to upload the new file.

never appears on the console, and I cannot establish an FTP connection to 192.168.1.1.

Is there any additional command required to enable the FTP recovery server ?

Thank you

Zyxel_Melen

Hi @ABIDAR

That's strange. Let me check with our engineer and update you next Monday.

Additionally, could you share some of device information? I will send you a private message since the question includes device's MAC address & S/N.

Thanks.

Zyxel_Judy

Hi @ABIDAR ,

Thank you for the information you shared via private message.

As you mentioned, the firewall suddenly became stuck in a boot loop, and you're not sure whether this may have been caused by a power outage. We recommend connecting the firewall to a stable power source and attempting the firmware recovery again.

If the firewall remains stuck in the boot loop, please contact your reseller/ISP to arrange an RMA.